Various projection-exposure apparatus are known such as steppers and the like. Such apparatus are typically used in microlithography processes for the manufacture of, e.g., semiconductor devices, and involve the transfer by projection of a pattern defined on a reticle (also termed a "photomask" or simply "mask") onto a suitable substrate, such as a silicon wafer or glass plate, having a photosensitive surface. Production of devices having ever greater feature density has required a corresponding increase in the projection resolution obtainable with such apparatus.
Some increased projection resolution has been achieved by use of increasingly shorter wavelengths of illumination light. Most current projection-exposure apparatus employ 365-nm i-line light generated by a high-pressure mercury lamp. Recently, excimer lasers have become more favored because of their high intensity and short wavelengths of light. For example, a KrF excimer laser produces 248-nm light, which is in the far ultraviolet range and has a substantially shorter wavelength than i-line light. Also, the ArF excimer laser (193 nm) is receiving greater attention as an illumination source.
As an alternative to excimer lasers, it is also possible to use a harmonic metallic vapor laser (e.g., copper-vapor laser) or a YAG laser, for example, to generate short-wavelength illumination light in the ultraviolet range. The fact that such lasers, as well as excimer lasers, are pulse lasers poses certain technical challenges.
Projection-exposure apparatus employ a projection lens to refract light passing through the reticle onto the surface of the substrate. Such projection lenses as used to refract ultraviolet light must have a high transmissivity to the light. The glass used to make such lenses is currently limited to quartz and fluorite.
When light from an excimer laser, especially from an ArF excimer laser, is used as the exposure illumination light, serious problems have been encountered in prior-art projection-exposure apparatus. As stated above, excimer lasers are pulsed. The pulses typically have a very rapid rise time, e.g., several nsec, at a frequency range of 600 to 1000 Hz. Consequently, the amount of energy per pulse required to obtain an exposure energy per unit time that is equivalent to the exposure energy produced by a source that generates continuous illumination is extremely large if throughput is to be maintained. The great and rapid change in instantaneous energy of such pulsed laser light has been found to cause a phenomenon called "solarization" of the projection lens. Solarization results in a lowering of the transmissivity of the incident area of the lens, even in lenses made of quartz. The extent of the decrease in transmissivity is proportional to the square of the light "fluence," wherein the fluence is the irradiation energy density per pulse of light mJ/(cm.sup.2 .multidot.pulse)!.
In addition, exposure to high-intensity pulsed ultraviolet light will cause quartz to experience, in the incidence area, a phenomenon called "radiation compaction." Radiation compaction causes the refractive index to increase at a rate proportional to the square of the fluence. In one example in which KrF excimer laser light was used with a quartz lens in which the fluence was 1 mJ/(cm.sup.2 .multidot.pulse), the increase in refractive index after 10.sup.10 pulses was approximately 5.times.10.sup.-7. The rate of radiation compaction is a function of wavelength. A similar exposure of quartz to an ArF excimer laser causes an increase in refractive index from radiation compaction that is approximately two orders of magnitude greater than when the quartz is exposed to a KrF excimer laser.
Radiation compaction can seriously degrade the ability of a projection lens to correct aberrations. In addition, solarization causes a lens to absorb more of the light irradiation energy as heat. Such localized heating can cause significant thermal deformations of the lens, thereby increasing aberrations. Such heating can also increase the refractive index of the lens, which can also increase aberrations.
It is known that solarization and radiation compaction can be lessened by increasing the pulse frequency of the illumination light and correspondingly lowering the light energy per pulse. For example, doubling the pulse frequency allows the light energy per pulse to be halved while obtaining the same throughput (number of wafers processed per unit time using the projection-exposure apparatus). That is, doubling the pulse frequency of the illumination light allows the fluence (F) to be halved.
More specifically, the number of illumination-light pulses per unit time (i.e., pulse frequency) is denoted by P. Both the solarization and radiation compaction are proportional to P.multidot.F.sup.2. Thus, with respect to an initial fluence of F.sub.0 and an initial pulse frequency of P.sub.0, changes in transmissivity and refractive index of a projection lens due to solarization and radiation compaction can be halved by doubling P.sub.0 and halving F.sub.0 : EQU F.sup.2 .multidot.P=(F.sub.0 /2).sup.2 (2P.sub.0)=F.sub.0.sup.2 (P.sub.0 /2)
In other words, at the same throughput, the effects of solarization and radiation compaction of a projection lens can be reduced in inverse proportion to a change in the pulse frequency.
Incidentally, fluorine gas (F.sub.2) used as the laser gas in a pulse-emitting excimer laser contains various impurities, such as oxygen (O.sub.2) and nitrogen (N.sub.2), carbon tetrafluoride (CF.sub.4), hydrogen fluoride (HF), and silicon tetrafluoride (SiF.sub.4), as well as carbon (C), nitrogen dioxide (NO.sub.2) , and various fluorine compounds created by electrical discharge. These impurity compounds degrade the laser gas and thus decrease laser output; the impurities also absorb laser light and contribute to fouling of the transmission window in the laser. Consequently, modern excimer lasers provide for rapid exhaust of any contaminated laser gas at each pulse.
In general, an increase in the pulse frequency of light produced by an excimer laser requires a corresponding increase in the rate of the laser's exhaust mechanism. For example, in order to double the pulse frequency of light from an excimer laser, the laser-gas exhaust mechanism must be able to operate at double normal speed, a further increase in pulse frequency would require a correspondingly further increase in the speed of the exhaust mechanism. However, with current technology, there is a limit on the speed at which conventional excimer-laser exhaust mechanisms can operate (about 1 kHz). This imposes a limit on the pulse frequency of illumination light obtainable with conventional projection-exposure equipment.